This invention relates to apparatus which when assembled is operable to reliquefy a compressed vapour, particularly apparatus which is operable on board ship to reliquefy natural gas vapour.
Natural gas is conventionally transported over large distances in liquefied state. For example, ocean going tankers are used to convey liquefied natural gas from a first location in which the natural gas is liquefied to a second location in which it is vaporised and sent to a gas distribution system. Since natural gas liquefies at cryogenic temperatures, i.e. temperatures below xe2x88x92100xc2x0 C., there will be continuous boil-off of the liquefied natural gas in any practical storage system. Accordingly, apparatus needs to be provided in order to reliquefy the boiled-off vapour. In such an apparatus a refrigeration cycle is performed comprising compressing a working fluid in a plurality of compressors, cooling the compressed working fluid by indirect heat exchange, expanding the working fluid, and warming the expanded working fluid in indirect heat exchange, and returning the warmed working fluid to one of the compressors. The natural gas vapour, downstream of a compression stage, is at least partially condensed by indirect heat exchange with the working fluid being warmed. One example of an apparatus for performing such a refrigerant method is disclosed in U.S. Pat. No. 3,857,245.
According to U.S. Pat. No. 3,857,245 the working fluid is derived from the natural gas itself and therefore an open refrigeration cycle is operated. The expansion of the working fluid is performed by a valve. Partially condensed natural gas is obtained. The partially condensed natural gas is separated into a liquid phase which is returned to storage and a vapour phase which is mixed with natural gas being sent to a burner for combustion. The working fluid is both warmed and cooled in the same heat exchanger so that only one heat exchanger is required. The heat exchanger is located on a first skid-mounted platform and the working fluid compressors on a second skid-mounted platform. Nowadays, it is preferred to employ a non-combustible gas as the working fluid. Further, in order to reduce the work of compression that needs to supplied externally, it is preferred to employ an expansion turbine rather than a valve in order to expand the working fluid.
An example of an apparatus which embodies both these improvements is given in WO-A-98/43029. Now two heat exchangers are used, one to warm the working fluid in heat exchange with the compressed natural gas vapour to be partially condensed, and the other to cool the compressed working fluid. Further, the working fluid is compressed in two separate compressors, one being coupled to the expansion turbine. Although not disclosed in WO-A-98/43029 this conventional apparatus is so installed on board ship that the heat exchangers and the compressor which is coupled to the expansion turbine are located in the cargo machinery room of the ship and the other compressor is located within the engine room. A need arises to simplify the machinery arrangements of such an apparatus.
According to the present invention there is provided apparatus which when assembled is operable to reliquefy a compressed vapour by a method comprising performing an essentially closed refrigeration cycle comprising compressing a working fluid in at least one compressor, cooling the compressed working fluid by indirect heat exchange in a first heat exchanger, expanding the cooled working fluid in at least one expansion turbine, warming the expanded working fluid by indirect heat exchange in a second heat exchanger, and returning the warmed expanded working fluid through the first heat exchanger to the said compressor, and at least partially condensing the compressed vapour in the second heat exchanger, wherein the apparatus comprises a first support platform on which a first pre-assembly including the second heat exchanger is positioned and a second support platform on which a second pre-assembly is positioned, characterised in that the said compressor, the said expansion turbine and the first heat exchanger are all included in the second pre-assembly.
By mounting the said compressor and the said expansion turbine on the same platform, they may both be located in the engine room, or a specially ventilated cargo motor room in the deck house, of an ocean going vessel on which the apparatus is to be used. In these locations the safety requirements that the compressor and the expansion turbine are required to meet are not as high as in other parts of the ship, for example an unventilated cargo machinery room. Thus, a useful simplification of the apparatus is provided. Further, by locating the compressor and the expansion turbine on the same platform, they can be incorporated into a single machine. If desired, the said compressor and said expansion turbine can be mounted on the same shaft, or, alternatively, they may all be operatively associated with the same gear box. Not only does employing a single compression/expansion machine simplify the apparatus, it also facilitates testing of the machinery prior to assembly of the apparatus according to the invention on board ship.
Preferably, all inter-and after- coolers associated with the said compressor are located on the second platform. This provides a further simplification over the known apparatus in which the compressors are located in separate parts of the ship requiring supplies of cooling water to both such parts.
The compression/expansion machine preferably includes no more than three compression stages.
Preferably the said compressor and the said expansion turbine employ seals of a kind which minimise leakage of working fluid out of the working fluid cycle. Accordingly, instead of conventional labyrinthine seals, either dry gas seals or floating carbon ring seals are used instead. Even so, it is desirable that the apparatus includes a source of make-up working fluid. By minimising the loss of working fluid, the amount of make-up working fluid that is required is similarly minimised. Since the working fluid is typically required at a pressure in the range of 10 to 20 bar (1000 to 2000 kPa) on the low pressure side of the cycle, this helps to keep down the size of any make-up working fluid compressor that might be required. If nitrogen is selected as the working fluid, it may alternatively become possible to employ a source of nitrogen which is already at the necessary pressure and thereby obviate the need for any make-up working fluid compressor whatever. For example, the source of the make-up nitrogen may be a bank of compressed nitrogen cylinders or, if the ship is provided with a source of liquid nitrogen, a liquid nitrogen evaporator of a kind that is able to provide gaseous nitrogen at a chosen pressure in the range of 10 to 20 bar. Such liquid nitrogen evaporators are well known.
Preferably there is a third pre-assembly comprising the make-up working fluid supply means on a third platform.
Preferably the platforms used in the apparatus according to the invention are skid-mounted.
Preferably, the first heat exchanger is located within a first insulated housing and the second heat exchanger is located in a second insulated housing.
Although the apparatus according to the invention is particularly suitable for use in reliquefying natural gas, it may be employed to reliquefy the vapour of other volatile liquids or organic compounds that are transported in a tank or tanks on board a ship, or are stored in a tank or tanks forming part of an on-shore or off-shore installation.